1. Technical Field
The present invention relates to a sintered material for valve guides that may be used in an internal combustion engine, and also relates to a production method for the sintered material for valve guides. Specifically, the present invention relates to a technique for further improving wear resistance of the sintered material for valve guides.
2. Background Art
A valve guide used in an internal combustion engine is a tubular component having an inner circumferential surface for guiding valve stems of an intake valve and an exhaust valve. The intake valve may be driven so as to take fuel mixed gas into a combustion chamber of the internal combustion engine, and the exhaust valve may be driven so as to exhaust combustion gas from the combustion chamber. For guiding the valve stems of the intake valve and the exhaust valve, the valve guide is required to have wear resistance and is also required to maintain smooth sliding conditions so as not to cause wear of the valve stems for long periods. Valve guides made of a cast iron are generally used, but valve guides made of a sintered alloy have recently come into wide use. This is because sintered alloys can have a specific metallic structure, which cannot be obtained from ingot materials, and therefore the sintered alloys can have wear resistance. Moreover, once a die assembly has been made, products having the same shape can be mass-produced, and therefore the sintered alloys are suitable for commercial production. Furthermore, a sintered alloy can be formed into a shape similar to that of a product, and thereby material yield can be high in machining. Valve guides made of a sintered alloy are disclosed in, for example, Japanese Examined Patent Publication No. 55-034858 and Japanese Patents Nos. 2680927, 4323069, and 4323467.
The sintered material for valve guides disclosed in Japanese Examined Patent Publication No. 55-034858 is made of an iron-based sintered alloy consisting of, by weight, 1.5 to 4% of C, 1 to 5% of Cu, 0.1 to 2% of Sn, not less than 0.1% and less than 0.3% of P, and the balance of Fe. A photograph and a schematic view of a metallic structure of this sintered material are shown in FIGS. 3A and 3B, respectively. As shown in FIGS. 3A and 3B, in this sintered material, an iron-phosphorus-carbon compound phase is precipitated in a pearlite matrix which is strengthened by adding copper and tin. The iron-phosphorus-carbon compound absorbs C from the surrounding matrix and grows into a plate shape, whereby a ferrite phase is dispersed at a portion surrounding the iron-phosphorus-carbon compound phase. Moreover, a copper alloy phase is dispersed in the matrix. The copper alloy phase is formed such that Cu is solved in the matrix during sintering at high temperature in an amount greater than the solid solubility limit at room temperature and is precipitated in the matrix by cooling. In the photograph of the metallic structure shown in FIG. 3A, since a graphite phase was exfoliated when the sample was polished so as to observe the metallic structure, the graphite phase cannot be observed. Nevertheless, as shown in the schematic view of FIG. 3B, graphite remains inside a large pore and is dispersed as a graphite phase. This sintered material has superior wear resistance due to the iron-phosphorus-carbon compound phase. Therefore, this sintered material has been mounted in automobiles and has been commercially used by domestic and international automobile manufacturers. In this case, this sintered material is used as a common material for valve guides for internal combustion engines in four-wheeled automobiles.
The sintered material for valve guides disclosed in Japanese Patent No. 2680927 is an improved material of the sintered material disclosed in Japanese Examined Patent Publication No. 55-034858. In this material, in order to improve machinability, magnesium metasilicate minerals and magnesium orthosilicate minerals are dispersed as intergranular inclusions in the metallic matrix of the sintered material disclosed in Japanese Examined Patent Publication No. 55-034858. As with the sintered material disclosed in Japanese Examined Patent Publication No. 55-034858, this sintered material has been mounted in automobiles and has been commercially used by domestic and international automobile manufacturers.
The sintered materials for valve guides disclosed in Japanese Patents Nos. 4323069 and 4323467 have further improved machinability. The machinabilities thereof are improved by decreasing amount of phosphorus. That is, the dispersion amount of the hard iron-phosphorus-carbon compound phase is decreased to only the amount that is required for maintaining wear resistance of a valve guide. These sintered materials for valve guides have been mounted in automobiles and have started to be commercially used by domestic and international automobile manufacturers.
Recently, requirements for reducing the production costs have been increasing for various industrial machine parts, and also the requirements for reducing the production costs have been increasing for automobile parts. In view of these circumstances, further reduction of the production costs is also required for sintered materials for valve guides for internal combustion engines.
In the meantime, in accordance with trends toward improving the performance and the fuel efficiency of automobile internal combustion engines in recent years, valve guides have been subjected to higher temperatures and higher pressures while internal combustion engines are running. Moreover, in view of recent environmental issues, amounts of lubricant supplied to an interface between a valve guide and a valve stem have decreased. Therefore, valve guides must withstand more severe sliding conditions. In view of these circumstances, a sintered material for valve guides is required to have high wear resistance equivalent to those of the sintered materials disclosed in Japanese Examined Patent Publication No. 55-034858 and Japanese Patent No. 2680927.